Some thoughts on fans and humidity switches (bath/shower rooms)

[Harry Bloomfield]

I keep hearing comments about humidity switches, for bathroom fans, not being very effective, yet I do find mine to be reasonably effective. I wonder if the reason might be lack of effective airflow, over some humidity sensor designs?

My bathroom fan, is a type which includes either PIR triggering, and/or humidity triggering, with an optional terminal to be triggered by the switching on of the light. The latter, I found to be entirely superfluous....

You walk in the room, and the PIR 'sees' you, switches the fan on, and seconds later the shutter opens, which creates a good flow of the soon to become, quite humid air, past the humidity sensor. Once triggered, I have it's timer, set to keep the fan running for a minimum of exactly 20minutes. If the bathroom air is not made humid, by the use of the bath, or shower, the fan switches off at exactly 20 minutes. If however, the bath, or shower are used, the fan will continue to run beyond the 20 minutes, until the higher level of humidity is cleared. Which is exactly as I would expect to happen. I have the sensor set for around 70%, just slightly higher than the background humidity in the house, but that took some careful tuning, to avoid it being triggered by the natural background humidity level.

A fan, only triggered by turning the light on, or built-in humidity sensor, cannot work reliably, because there is no certainly that it will sense the level of humidity, unless the light is turned on. So far as I am concerned, then an effective PIR, is essential.

 

[ericmark]

I believe you are right, it needs a PIR as well as humidity detector. I had a humidity meter, 0 - 90% worked reasonably well over 90% it would get water logged and would need placing in doors for a week or two to dry out, we were looking at the humidity of a woodland.

Problem in a bathroom, once wet to over 90% very little chance of drying out.

 

[Harry Bloomfield]

Problem in a bathroom, once wet to over 90% very little chance of drying out.

That is not my experience. Many appear to work, using a conductive when moist film. A comparator, compares the resistance of the film, to a preset value.

 

[flameport]

For any meaningful results, you need at least 2 separate humidity sensors. Such as one for the extracted air, and another for air elsewhere in the property, or in the case of MVHR systems air which has been taken from outside and will be put back into the property.

Otherwise it's a single sensor and the extractor operates based on some arbitrary setting, which even if valid today won't be in a month or four when conditions change.

 

[AndyPRK]

They do require a lot of fine adjustment and some way of turning on the fan at the start of the shower.

Not really something your electrican can do.

 

[Harry Bloomfield]

Otherwise it's a single sensor and the extractor operates based on some arbitrary setting, which even if valid today won't be in a month or four when conditions change.

Ideally, yes, but in practice no - mine never triggers except with the high humidity of the use of the shower, or a bath. I continuously record both the indoor and outdoor humidity in my home. Indoor at the moment, and for several days, has been around 60%, as measured on the landing, near the bathroom, with brief excursions up towards 70%, when baths/showers have been taken. Exceptionally, it rose to 72% today, during the Christmas dinner cooking. Outdoors, is showing between 78 and 86%.

Bathroom fan is not being triggered, because the bathroom humidity is lower than the landing area.